Diffusion Tensor Imaging and Chemical Exchange Saturation Transfer MRI Evaluation on the Long-Term Effects of Pulsed Focused Ultrasound and Microbubbles Blood Brain Barrier Opening in the Rat.

Tsang-Wei Tu,William C Reid,Maggie Sundby,Zsofia I Kovacs,Joseph A Frank,Dima A Hammoud,Georgios Z Papadakis,Jaclyn A Witko

doi:10.3389/fnins.2020.00908

Abstract

Blood-brain barrier opening (BBBO) with pulsed Focused Ultrasound (pFUS) and microbubbles (MB) has received increasing interest as a method for neurotherapeutics of the central nervous system. In general, conventional MRI [i.e., T2w, T2∗w, gadolinium (Gd) enhanced T1w] is used to monitor the effects of pFUS+MB on BBBO and/or assess whether sonication results in parenchymal damage. This study employed multimodal MRI techniques and 18F-Fludeoxyglucose (FDG) PET to evaluate the effects of single and multiple weekly pFUS+MB sessions on morphology and glucose utilization levels in the rat cortex and hippocampus. pFUS was performed with 0.548 MHz transducer with a slow infusion over 1 min of OptisonTM (5–8 × 107 MB) in nine focal points in cortex and four in hippocampus. During pFUS+MB treatment, Gd-T1w was performed at 3 T to confirm BBBO, along with subsequent T2w, T2∗w, DTI and glucose CEST (glucoCEST)-weighted imaging by high field 9.4 T and compared with FDG-PET and immunohistochemistry. Animals receiving a single pFUS+MB exhibited minimal hypointense voxels on T2∗w. Brains receiving multiple pFUS+MB treatments demonstrated persistent T2w and T2∗ abnormalities associated with changes in DTI and glucoCEST when compared to contralateral parenchyma. Decreased glucoCEST contrast was substantiated by FDG-PET in cortex following multiple sonications. Immunohistochemistry showed significantly dilated vessels and decreased neuronal glucose transporter (GLUT3) expression in sonicated cortex and hippocampus without changes in neuronal counts. These results suggest the importance to standardize MRI protocols in concert with advanced imaging techniques when evaluating long term effects of pFUS+MB BBBO in clinical trials for neurological diseases.

Highlights

MRI guided (MRIg) pulsed Focused Ultrasound is a noninvasive technique being advocated for the blood brain barrier opening (BBBO) to facilitate the delivery of neurotherapeutics in the treatment of primary and metastatic central nervous system (CNS) tumors (Park et al, 2012, 2020) or neurodegenerative diseases, such as amyotrophic lateral sclerosis (Abrahao et al, 2019) and Alzheimer’s disease (Baseri et al, 2012; Kovacs et al, 2014; Lipsman et al, 2018). pFUS coupled with an infusion of ultrasound contrast agent microbubbles (MB) causes BBBO primarily by mechanical effects from acoustic cavitation forces on the endothelium that alters the integrity of tight junction proteins (TJP) and changes calcium fluxes within vasculature (Sheikov et al, 2004, 2008; Deng, 2010; Burgess and Hynynen, 2014; Gorick et al, 2020)
In the animals receiving multiple weekly pFUS+MB, there was a decrease in the image contrast on the glucoCESTweighted imaging and FDG-positron emission tomography (PET) in the cortex when compared to unsonicated animals or the Group 1 rats sonicated once in week 6
Various pFUS+MB protocols have been used in both experimental and clinical studies that result in BBBO, facilitating the delivery of neurotherapeutics to targeted regions in the brain 2017; Neuwelt et al, 2008; Gabathuler, 2010; Burgess et al, 2014; and potentially enhancing neurological outcomes (Kroll and Chai et al, 2014; Horodyckid et al, 2017; Kovacs et al, 2017b, Neuwelt, 1998; Hynynen et al, 2001; McDannold et al, 2008, 2018a; McMahon et al, 2017; Wu et al, 2017)

Summary

Introduction

MRI guided (MRIg) pulsed Focused Ultrasound (pFUS) is a noninvasive technique being advocated for the blood brain barrier opening (BBBO) to facilitate the delivery of neurotherapeutics (i.e., drugs, genes, biologics) in the treatment of primary and metastatic central nervous system (CNS) tumors (Park et al, 2012, 2020) or neurodegenerative diseases, such as amyotrophic lateral sclerosis (Abrahao et al, 2019) and Alzheimer’s disease (Baseri et al, 2012; Kovacs et al, 2014; Lipsman et al, 2018). pFUS coupled with an infusion of ultrasound contrast agent microbubbles (MB) causes BBBO primarily by mechanical effects from acoustic cavitation forces on the endothelium that alters the integrity of tight junction proteins (TJP) and changes calcium fluxes within vasculature (Sheikov et al, 2004, 2008; Deng, 2010; Burgess and Hynynen, 2014; Gorick et al, 2020). Gadoliniumbased contrast agents (GBCA)-T1-weighted (w) MRI are used to document BBBO following sonication, confirmed by extravasation of dyes or plasma proteins into the parenchyma on histology (Yang et al, 2009; Chai et al, 2018; Stavarache et al, 2018; Morse et al, 2019). The level of contrast enhancement in GBCA-T1w MRI may be related to the magnitude of sonication parameters and the amount of CCTF expression (Nance et al, 2014; Mooney et al, 2016; Olumolade et al, 2016; Kovacs et al, 2017b; McMahon et al, 2017). Immunohistochemistry (IHC) techniques have been used to document changes in cellular activation following sonication and BBBO (Hynynen et al, 2001; Aryal et al, 2015).

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